What goes into Soda bread is a subject of great debate. http://www.sodabread.us/argues that it should (if it is to be traditional) only have flour, salt, baking soda, and buttermilk. The baking soda reacts with the sour buttermilk to produce the leavening. It is, in a sense, biscuits without the fat.

Sugar, currants and butter move it into cake territory. They also give it a better shelf life, and to modern taste make it more interesting.